Compaction measurement is of great significance in the construction of highways, airports, railway embankments, trench backfills, dams and foundations. The knowledge of material density can be a major indicator of how well a resident structure will perform its intended usage. Under compaction can cause serious deformation and settlement of the structure, while over compaction can cause cracks that affect the required material strength. Also, in-place density measurements are necessary to ensure proper testing of asphalt paving used in highways, airports and parking lots. Either by design choice or to comply with standards and/or job requirements, density measurements are used as a field quality control test for monitoring the compaction of soil, asphalt and concrete structures.
Nuclear gauges are the standard method of density measurement in most heavy construction projects. Various State Departments Of Transportation (DOT), as well as the Federal Highway Administration (FHWA), have adopted specifications for use of nuclear density gauges. Nuclear gauges are used to determine compliance with the specification for construction projects. While, there are numerous testing methodologies in use to measure structure density, nuclear testing devices are the preferred standard around the world, due to their speed, accuracy, and convenience.
Older, more primitive testing methods such as sand cone (soils), balloon (soils) and core samples (asphalt) testing are time-consuming and involve taking samples of the test materials off-site for analysis. Results are often not available for as long as 24 hours after sampling, which is especially problematic in asphalt construction projects. These testing processes are labor-intensive and necessarily involve the destruction of small pieces of the structure material. Nuclear gauges, on the other hand, are portable devices which are placed on the material and automatically display the material density in as little as 15 seconds.
Nuclear Density Gauge Theory
Nuclear density gauges operate by using a very small radioactive source and a detection system. When placed on the test material, the photons from the nuclear source penetrate the material. A fraction of the photons will interact with the material and scatter to the gauge base where they are detected by Geiger Mueller detectors. The number of photons scattered back and counted by the detectors is proportional to the material density.
Nuclear gauges can operate in two different modes: backscatter and direct transmission. In the backscatter mode, the gauge is placed on the test material with the source and the detectors in the same plane. The photons from the source penetrate the material from the surface and scatter to the detectors. This mode is normally used for measurement of asphalt and hardened concrete. Hence, no destruction of the test structure occurs. In the direct transmission mode, a hole larger than the diameter of the source rod is formed in the material (normally soils) and the source is inserted into the material at a predetermined depth. In this mode, the material is located directly in the path between the source and the detectors.
Nuclear density gauges are also widely used in the processing industry for measurement of raw material density and automatic control of process operations. Even though the design of the radioactive source material and detectors are different than the ones used in the construction industry, the principles of density measurement are the same as discussed above. Therefore, the teachings of this invention can be used for the calibration validation and calibration of nuclear density devices in all other industries.
Calibration of Nuclear Gauges
As with many testing instruments, calibration is of vital importance for nuclear gauges. Presently, nuclear gauges are calibrated in the factory by using blocks of known density. These blocks are large (i.e. 24"L, 17"W, 14"H) and heavy (360 to 560 lbs). The blocks often consist of metals (i.e. magnesium, aluminum, and/or a combination block of magnesium and aluminum) and natural materials such as limestone and granite. Gauges are placed on these blocks and a "count" is obtained at each depth. Gauge measurements may involve counts obtained at depths of 0 inches (backscatter) to 12 inches (direct transmission), as well as intermediate depths at increments of 0.5, 1 or 2 inches, depending on the gauge design. These counts, along with the known block densities, are used in an equation such as EQU CR=A e.sup.(-BD) -C (1)
where:
CR is a count ratio or ratio of gauge count on the test material and a standard count PA1 A, B and C are gauge parameters for each depth PA1 D is the material density
The standard count is collected with the gauge on a small polyethylene block provided with each gauge. The standard count corrects the gauge counts for source decay (i.e. approximately 2.4% each year for Cesium-137) and minor electronic drift. The constants A, B and C are determined by a suitable curve fitting process using mathematical and computer methods well known in the art. These values are entered into the gauge memory for calculation of density in the field. Thus, each gauge receives values for the constants A, B and C for each depth from the factory calibration process.
Field Density Measurements
In the field, a count reading is taken on a test material. This reading is used, along with the A, B, and C constants for each depth and the standard count, to calculate a density using the rearranged equation (1) from above: ##EQU1## Calibration Standards and Requirements
Two ASTM standards that currently require validation of the nuclear gauge calibration are as follows: ASTM standard D2922, "Density Of Soil and Soil-Aggregate In Place By Nuclear Methods (Shallow Depth)" and ASTM Standard D2950, "Standard Test Method For Density Of Bituminous Concrete In Place By Nuclear Methods." ASTM D2922, requires that nuclear gauge calibration be verified at least once every 12-18 months. ASTM D2950 requires a verification of gauge calibration at least once per year. While the standards set forth the intervals for calibration and verification, there is little instruction on how and with what type of device the verification can be performed.
Nuclear gauges are calibrated with large and expensive density standards before leaving the factory. Each manufacturer uses a slightly different calibration method. The calibration is performed to establish the count to density relationship for each unit produced. Calibration enables the gauge to measure field material density.
Gauges are typically used in a very rough construction environment. Presently, two questions are commonly raised regarding the gauge calibration validity. The first question concerns how long a gauge will operate without a need for new calibration. The second question concerns the possible variation in density measurements of two gauges on a given material. The two gauges can be of the same make and model, or from different manufacturers. Without a reliable validation device, it is impossible to determine if one or both gauges require a new calibration. The answer to these questions presently can only be obtained by returning the gauges to the manufacturer or other testing laboratories equipped with calibration standards for verification and calibration of the gauges at all depths of operation. In addition to the costs charged by such entities to perform the new calibration (in effect, a re-calibration), the shipping costs and loss of use (an average of two weeks) can be substantial as well. Therefore, because of the inconveniences of the calibration techniques used today, users and manufacturers have yet to be provided with suitable tools to answer the question of whether and how often their gauge needs calibration. Thus, the absence of suitable validation and calibration techniques have hindered users who desire feedback that they are getting consistent and accurate results.
A small number of gauge owners have used two methods for gauge calibration validation. Some have molded blocks out of concrete for validation purposes. However, blocks made from construction materials are heavy and cannot be transported easily from site to site. Also, the block density can change, due to wear and the degree of moisture absorption over time. This change in block density defeats the usefulness of these blocks, since they cannot be relied on as a fixed density reference to validate the gauge calibration. Also, in some instances, a fixed location is marked on concrete floors or asphalt parking lots for gauge validation. Gauges are placed on this spot from time to time to test the validity of the calibration. The measurements under this condition are affected by the change in the material density over time and limited to the backscatter mode only. Also, validation at one depth cannot be used to predict accurately gauge calibration validity at other depths.
Prior art methods of calibration include those described in U.S. Pat. Nos. 4,587,623 and 4,791,656. U.S. Pat. No. 4,587,623, of which applicant is a co-inventor, uses the constant B from the original factory calibration and actual gauge counts on the magnesium and aluminum standard block to calculate the parameters A and C of equation (1) above. In this process, curve fitting is not necessary. The block densities, the B parameter and the counts can be plugged into the calibration equation and A and C are analytically calculated. The two blocks used for performing this calibration are large, heavy, expensive and not portable.
U.S. Pat. No. 4,791,656, requires the purchase of three blocks. Namely a magnesium block, a magnesium/aluminum (Mg/Al) combination block and an aluminum block. This method relies on the counts obtained from the Mg/Al combination block and historical relationships developed between Mg/Al and magnesium, and Mg/Al and aluminum blocks, of known densities to calculate counts for magnesium and aluminum, at all depths. These three counts are then used with an appropriate fitting routine to determine the parameters A, B and C. The method in the '656 patent requires an initial purchase of three blocks and a collection of data for a predetermined period. The data collected is then used to develop linear relationships between Mg/Al and magnesium, and Mg/Al and aluminum. After a determination of these historical relationships, this method provides an efficient calibration method for the gauge manufacturer. However, it is not a field practical and cost effective solution for the gauge users due to the requirement of multiple reference blocks.
Gauge users need a convenient and portable validation block that can be used as a fixed density reference and one that does not change with time. Also, if it is shown that the calibration cannot be validated, it is desirable to have a simple calibration process so that gauges can be calibrated, without the costly and cumbersome processes of shipping the gauge off-site for calibration.